The spatial local accuracy of land cover datasets over the Qiangtang Plateau,High Asia

We analyzed the spatial local accuracy of land cover (LC) datasets for the Qiangtang Plateau, High Asia, incorporating 923 field sampling points and seven LC compilations including the International Geosphere Biosphere Programme Data and Information System (IGBPDIS), Global Land cover mapping at 30 m resolution (GlobeLand30), MODIS Land Cover Type product (MCD12Q1), Climate Change Initiative Land Cover (CCI-LC), Global Land Cover 2000 (GLC2000), University of Maryland (UMD), and GlobCover 2009 (Glob-Cover). We initially compared resultant similarities and differences in both area and spatial patterns and analyzed inherent relationships with data sources. We then applied a geographically weighted regression (GWR) approach to predict local accuracy variation. The results of this study reveal that distinct differences, even inverse time series trends, in LC data between CCI-LC and MCD12Q1 were present between 2001 and 2015, with the exception of category areal discordance between the seven datasets. We also show a series of evident discrepancies amongst the LC datasets sampled here in terms of spatial patterns, that is, high spatial congruence is mainly seen in the homogeneous southeastern region of the study area while a low degree of spatial congruence is widely distributed across heterogeneous northwestern and northeastern regions. The overall combined spatial accuracy of the seven LC datasets considered here is less than 70%, and the GlobeLand30 and CCI-LC datasets exhibit higher local accuracy than their counterparts, yielding maximum overall accuracy (OA) values of 77.39% and 61.43%, respectively. Finally, 5.63% of this area is characterized by both high assessment and accuracy (HH) values, mainly located in central and eastern regions of the Qiangtang Plateau, while most low accuracy regions are found in northern, northeastern, and western regions.

     

中国传统农区过去300年耕地重建结果的对比分析

土地覆被变化是气候与生态效应模拟研究的重要参量。SAGE和HYDE两个全球历史土地利用数据集在相关研究中得到广泛应用, 但在区域尺度上的应用, 其可靠性如何, 至今少有论及。以我国学者重建的传统农区历史耕地数据集(CHCD) 为基础, 从全区、省区和网格(60 km×60 km) 三个空间尺度, 对SAGE (2010) 和HYDE3.1 数据集中有关中国传统农区历史耕地重建结果进行对比分析, 结果表明:(1) SAGE (2010) 数据集对中国传统农区耕地数量重建是以单一线性插补而得, 其中1700-1950 年是以0.51%的年均增长率线性递增, 1950 年后是以0.34%年均速率线性递减, 这种“标准化”变化趋势不能客观反映传统农区土地垦殖的真实历史, 耕地面积也明显高估, 与CHCD数据集不具有可比性;(2) HYDE3.1 数据集吸纳了区域性研究成果, 使其在总量上与CHCD数据集较为接近, 具有较好的可比性, 但其在省区和网格尺度上与CHCD存在显著差异, 其中相对差异率超过70% (< -70%或> 70%) 的网格占比高达56%~63%, 超过90% (< -90%或> 90%) 的网格占比也高达40%~45%;而相对差异率介于-10%~10%的网格占比仅为5%~6%, 介于-30%~30%的网格占比也仅为17%左右;(3) 充分利用我国丰富的历史文献, 建立更高精度的中国区域历史土地利用数据集, 是提高区域气候与生态效应模拟研究质量的重要保障。     

Mapping cropping intensity trends in China during 1982–2013

Long range continuous monitoring information of cropping intensity is useful for sustainable agricultural management but still limited. This study filled this information gap through delivering spatiotemporal continuous datasets of cropping intensity in China during the past 30 years. Cropping intensity data were derived by a wavelet features-based method based on the long-term weekly global EVI2 (Enhance Vegetation Index with two bands) at 0.05° spatial resolution (5 km) from 1982 to 1999 and 8-day composite 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance products from 2001 to 2013. The remote-sensing estimated images in 2013 agreed well with field survey data (overall accuracy = 91.63%) and the national agricultural census data (r² = 0.89). Results revealed that the cropping intensity remarkably increased during 1982–1999 but slightly declined during 2001–2013. The overall cropping intensity increased from 1.34 in the 1980s to 1.41 in the 1990s, and then dropped to an average of 1.36 after 2000. From 1982 to 1999, approximately 93,225 km² single-cropped areas changed to double-cropping, primarily those located in the North China plain. However, 39,883 km² double-cropped areas were turned back into single-cropping areas from 2001 to 2013, principally located in the North China plain, the Middle-lower Yangtze River plain, and the hill regions of the southern Yangtze River. This reverse trend of cropping intensity was due to combined effects from the corresponding reverse variations in agricultural population, increasing agricultural mechanical power, positive agricultural policy. The agricultural duty free policy has only immediate effects on stabilizing cropping intensity in croplands with more favorable biophysical conditions.     

A CRPS-Based Spatial Technique for the Verification of Ensemble Precipitation Forecasts

Traditional precipitation skill scores are affected by the well-known“double penalty”problem caused by the slight spatial or temporal mismatches between forecasts and observations. The fuzzy (neighborhood) method has been proposed for deterministic simulations and shown some ability to solve this problem. The increasing resolution of ensemble forecasts of precipitation means that they now have similar problems as deterministic forecasts. We developed an ensemble precipitation verification skill score, i.e., the Spatial Continuous Ranked Probability Score (SCRPS), and used it to extend spatial verification from deterministic into ensemble forecasts. The SCRPS is a spatial technique based on the Continuous Ranked Probability Score (CRPS) and the fuzzy method. A fast binomial random variation generator was used to obtain random indexes based on the climatological mean observed frequency, which were then used in the reference score to calculate the skill score of the SCRPS. The verification results obtained using daily forecast products from the ECMWF ensemble forecasts and quantitative precipitation estimation products from the OPERA datasets during June-August 2018 shows that the spatial score is not affected by the number of ensemble forecast members and that a consistent assessment can be obtained. The score can reflect the performance of ensemble forecasts in modeling precipitation and thus can be widely used.     

中国冬季最低气温观测与再分析资料的比较

运用EOF分解、线性趋势分析、方差分析、小波分析、相关分析等方法,从均值、长期变化、年际和年代际变化特征等方面对NCEP/NCAR(N1T)、NCEP/DOE(N2T)、ERA-Interim(ET)、JRA-25(JT)4种再分析资料集和194站观测资料集中1979—2012年我国冬季最低气温(WTMIN)进行比较。结果表明:(1)ET在再现观测冬季最低气温多年平均时最优;(2)除新疆北部和西南部以及东北北部有降低趋势外,我国大部分地区WTMIN有升高的趋势,N1T对观测温度长期变化趋势再现能力优于其他3种资料;(3)观测WTMIN在中国北方和高原地区年际和年代际变化大于南方地区,四种再分析WTMIN对我国冬季最低气温的年际和年代际变化的再现在不同区域各有所长,总体上以ET为优。(4)再分析资料在再现观测WTMIN年代际时间尺度上的能力相当,以ET最优,尤其是在我国中东部地区和新疆北部地区。N1T和N2T对东北地区观测冬季最低气温年际变化的再现能力较好,ET再现观测WTMIN在1990s之前,2005年之后的3~4 a变化周期特征时能力略优于其他3种资料。     

An Evaluation of Temperature and Precipitation Surface-Based and Reanalysis Datasets for the Canadian Arctic, 1950–2010

The spatial and temporal consistency of seasonal air temperature and precipitation in eight widely used gridded observation-based climate datasets (CANGRD, CRU-TS3.1, CRUTEM4.1, GISTEMP, GPCC, GPCP, HadCRUT3, and UDEL) and eight reanalyses (20CR, CFSR, ERA-40, ERA-Interim, JRA25, MERRA, NARR, and NCEP2) was evaluated over the Canadian Arctic for the 1950–2010 period. The evaluation used the CANGRD dataset, which is based on homogenized temperature and adjusted precipitation from climate stations, as a reference. Dataset agreement and bias were observed to exhibit important spatial, seasonal, and temporal variability over the Canadian Arctic with the largest spread occurring between datasets over mountain and coastal regions and over the Canadian Arctic Archipelago. Reanalysis datasets were typically warmer and wetter than surface observation-based datasets, with CFSR and 20CR exhibiting biases in total annual precipitation on the order of 300?mm. Warm bias in 20CR exceeded 12°C in winter over the western Arctic. Analysis of the temporal consistency of datasets over the 1950–2010 period showed evidence of discontinuities in several datasets as well as a noticeable increase in dataset spread in the period after approximately 2000. Declining station networks, increased automation, and the inclusion of new satellite data streams in reanalyses are potential contributing factors to this phenomenon. Evaluation of trends over the 1950–2010 period showed a relatively consistent picture of warming and increased precipitation over the Canadian Arctic from all datasets, with CANGRD giving moistening trends two times larger than the multi-dataset average related to the adjustment of the station precipitation data. The study results indicate that considerable care is needed when using gridded climate datasets in local or regional scale applications in the Canadian Arctic.     

Long-term precipitation and stream discharge records at seven forested experimental watersheds along a latitudinal transect in Japan: Jozankei,Kamabuchi, Takaragawa,Tsukuba, Tatsunokuchi-yama,Kahoku and Sarukawa

This data note introduces a database of long-term daily total precipitation and stream discharge data for seven forested watersheds in Japan that have been continuously monitored by the Forestry and Forest Products Research Institute. Three of the watersheds started data collection in the 1930s. Forest cover across the sites ranges from cool to warm temperate regions with the latitude spanning from 31 to 44° N and annual precipitation ranging from 1200 to 3000 mm yr⁻¹. The effects of vegetation change via clearcutting, thinning and forest fire (among other stressors) on stream discharge can be analysed from the long-term observation sites. Moreover, this multi-site dataset allows for inter- and intra-site comparisons of annual water loss (difference of annual precipitation and stream discharge). These long-term datasets can provide comprehensive insights into the effects of climate change and other stressors on forested ecosystems, not only in Japan but across a spectrum of forest types, if combined with other long-term records from other forested watersheds across the world.     

江淮流域极端降水时空变化特征:站点观测和再分析的对比

利用淮河流域1979—2011年260个站点观测、ERA-Interim和NCEP/DOE再分析资料的日降水量数据,选用8个极端降水指数,从空间分布、发展趋势、时间变化等方面对比分析了我国江淮流域极端降水的变化规律,研究了再分析数据的适用性,结果表明:1)持续湿润指数(CWD)、强降水日数(R10mm,R20mm)以及百分位指数(R95p,R99p)具有一致的北少南多的分布特征,而持续干燥指数(CDD)为北多南少,且强度指数(Rx1day,Rx5day)和百分位指数在浙江沿海均有极大值存在。2)大部分地区的强降水日数呈减少趋势,仅在江淮周边地区有弱上升趋势。3)区域平均的降水强度指数具有上升的趋势变化,逐月变化具有先增长后减少的结构特征,5—6月的增长量最大,峰值出现在7月,在夏末、冬季有较明显的随年代增加的趋势,在秋季则随年代减少。4)再分析资料ERA-Interim和NCEP/DOE对不同指数的再现能力有所不同,ERA-Interim对强降水日数(R10mm)、CDD、百分位指数的空间分布以及CDD的变化趋势再现能力较好,与强度指数和百分位指数年际变化的相关性较高,但对CWD变化趋势分布特点的再现能力较弱;NCEP/DOE更善于再现较强降水日数(R20mm)的空间分布以及强度指数和百分位指数的线性变化趋势。5)两种再分析资料能合理地再现强降水日数(R10mm,R20mm)和CDD年际变化特征和强度指数的季节变化特征。